The present invention relates generally to gas turbine engines, and, more specifically, to turbine nozzles therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases that flow downstream through high and low pressure turbines which extract energy therefrom. The high pressure turbine powers the compressor, and the low pressure turbine typically powers a fan disposed upstream from the compressor in a typical aircraft engine application.
Engine performance and efficiency are increased by increasing the temperature of the combustion gases from which energy is extracted in the turbines. This is particularly important for military turbofan engines in which maximum performance is desired at the expense of engine life and fuel economy.
The combustion gases are first discharged through a high pressure turbine nozzle which direct the gases through high pressure turbine blades for energy extraction therefrom, with the gases losing temperature as they flow downstream through the engine. The high pressure turbine nozzle therefore experiences the maximum gas temperature and is typically formed of high strength superalloy materials for withstanding the hot gas environment, and is cooled using air bled from the compressor.
However, state of the art superalloy metal materials have limited temperature capability for ensuring a suitable life thereof. And, the ability to cool the turbine nozzle is also limited by the cooling ability of the air bled from the compressor.
Accordingly, further advances in high performance aircraft engines require further improvements over conventional superalloy metal materials and cooling thereof. One promising advancement is the use of ceramic material in turbine nozzles for its inherent high temperature capability and thermal insulating capability. The ceramic material may either be monolithic or reinforced with ceramic fibers commonly referred to as Ceramic Matrix Composite (CMC). Such CMC ceramic materials are presently being developed and commercially available from various sources in the exemplary form of Silicon Carbide (SiC) in which a matrix of SiC is reinforced with SiC fibers.
However, ceramic materials are brittle and lack ductility and can withstand very little strain prior to failure.
Correspondingly, typical nickel based superalloys are ductile and can accommodate substantial strain. This is significant in the design of turbine nozzles subject to hot combustion gases and lower temperature cooling air which develop substantial thermal strain during operation.
Ceramic nozzle design therefore must be suitably tailored to minimize thermally induced strain due to the mechanical assembly of nozzle components as well as due to cooling thereof. For example, a typical metal turbine nozzle includes impingement baffles inside the individual nozzle vanes having a multitude of impingement holes which direct the cooling air perpendicularly against the inner surface of the vanes for impingement cooling thereof.
Impingement cooling is quite effective for removing heat, and correspondingly creates a substantial difference in temperature between the relatively cool inner surface of the vane wall and the relatively hot outer surface of the vane wall. The corresponding thermal strain from this differential temperature across the vane wall is accommodated by the ductility of typical nickel based superalloy metal material used in conventional turbine nozzles.
A ceramic turbine nozzle exasperates this problem since the ceramic is a thermal insulator. A ceramic nozzle vane wall would then experience an even greater temperature differential thereacross from impingement cooling on the inside thereof with the hot gases on the outside thereof. The increased differential temperature across a ceramic nozzle wall will create a corresponding increase in thermal strain, and due to the brittle nature of ceramic will substantially shorten the useful life thereof.
Accordingly, it is desired to provide an improved cooling arrangement for a ceramic turbine nozzle for limiting thermally induced strain therein due to differential temperature.
A turbine nozzle baffle includes a plurality of apertures inclined through a shell for discharging a coolant tangentially therefrom. The baffle is disposed inside a nozzle vane and the tangentially discharged coolant flows generally parallel along the inner surface of the vane for convection cooling thereof.